In the hydrocarbon processing industry, it is common to recover valuable products from the heavy residual oil that remains after refining operations are completed. This recovery process produces a byproduct called coke and is called coking. The coking process is performed in large vessels with top and bottom head covers called coke drums. The bottom head covers are typically circular steel plates that are often 6 feet in diameter and bolted onto a flange at the bottom of the coke drum. The top head covers are similar, but often 36 inches in diameter. A gasket, such as a double-soft iron-jacketed non-asbestos filled gasket, is placed between the coke drum top and bottom flanges and top and bottom head covers to help provide a vapor-tight seal for the coke drum. Typically, these flanges and covers are designed by the rules of American Society of Mechanical Engineers Sect. VIII Div 1, Appendix II, or other vessel code.
During the coking process, the drums are subjected to high temperatures (typically 900 to 930 degrees Fahrenheit at the inlet) and pressures (typically 15 to 50 psig overhead pressure). Thus, the coke drums have to be steam purged and cooled with quench water prior to removal of the coke. After the drums are vented to atmosphere and quench water is drained, the top and bottom head covers are unbolted and removed or “unheaded.” Due to the hazard of hot water and coke fallout, there are risks associated with workers manually removing the head covers. At some sites, head removal is done automatically by costly and complicated, fully remotely actuated equipment. However, most cokers are still outfitted with either fully manual systems where the bottom head covers are removed by carts or trolleys, or hydraulically actuated systems that swing the head covers out of the way. These more common systems still require the bolts on the head covers to be manually loosened and removed. The typical procedure is to have the bottom head handling device push up against the bottom head cover to the coke drum while workers using air wrenches loosen up to 70 or more bolts per head cover. Then, after the bolts have been loosened, the bottom head cover may be actuated remotely to remove it from the coke drum using automated equipment.
While the bolts are being loosened, however, extremely hot (e.g., 400 degrees Fahrenheit or higher) water, steam and coke may escape if the seal between the bottom head cover and the coke drum is not maintained. The hot water and steam escaping under pressure may cause serious burns and other injuries to the workers loosening the bolts. Consequently, care must be taken when removing the bolts to make sure that the seal of the coke drum is not compromised.
A number of coke drum bottom unheading systems exist. For example, U.S. Pat. Nos. 5,876,568 and 6,066,237 use a specially designed clamp to hold the bottom head cover against the drum flange. The drum flange has a tapered top and the clamp is in the form of a ring that has been cut into two or more equal segments around the diameter. As clamping force is applied tangentially to the outside of the ring clamp segments, movement of the clamp up the tapering at the top of the bottom flange tightens the clamp and provides the additional pressure needed to hold the bottom head to the drum.
Another system that uses clamps to support the bottom head cover during unheading is described in U.S. Published Patent Application No. 20020157936. This system employs a cart having vertical supports for four or more clamps that can be rotated around the vertical supports. The clamps are applied after removing one-half of the head cover bolts (every other bolt), and hydraulic pressure is then used to hold the bottom head cover in place while the remaining bolts are removed.
These and other existing unheading systems, however, tend to be mechanically complex and/or very expensive (e.g., $1 million per coke drum). Moreover, the semi-manual systems known to be available commercially today provide insufficient clamping force to reliably maintain the gasket load between the head covers and the coke drum all the way around the perimeter. Accordingly, what is needed is a coke drum clamping system and method that is mechanically simple, reliable, and relatively inexpensive, and that provides protection for workers removing the bolts against the risk of hot vapor and/or water hazards associated with the work.